Method for determining the concentration of an analyte in a liquid sample using small volume samples and fast test times

ABSTRACT

Analytes in a liquid sample are determined by methods utilizing sample volumes of less than about 1.5 μl and test times within ten seconds. The methods are preferably performed using small test strips including a sample receiving chamber filled with the sample by capillary action.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/677,737 filed Feb. 22, 2007 (attorney docket no. 7404-787), which isa continuation of U.S. patent application Ser. No. 10/382,322 filed Mar.5, 2003 (attorney docket no. 7404-473) which is a continuation-in-partof U.S. patent application Ser. No. 10/264,785 filed Oct. 4, 2002(attorney docket no. 7404-426) which claims the benefit of U.S.Provisional Patent Application No. 60/332,411 filed Nov. 16, 2001(attorney docket no. 7404-425), which are hereby incorporated byreference in their entirety. This application is also acontinuation-in-part of U.S. patent application Ser. No. 10/264,891filed Oct. 4, 2002 (attorney docket no. 7404-431) which claims thebenefit of U.S. Provisional Patent Application No. 60/332,411 filed Nov.16, 2001 (attorney docket no. 7404-425), which are hereby incorporatedby reference in their entirety. This application is also acontinuation-in-part of U.S. patent application Ser. No. 10/264,785filed Oct. 4, 2002 (attorney docket no. 7404-426) which claims thebenefit of U.S. Provisional Patent Application No. 60/332,411 filed Nov.16, 2001 (attorney docket no. 7404-425), which are hereby incorporatedby reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to methods for determining theconcentration of an analyte in a liquid sample, and particularly tomethods using sample volumes of less than 1.5 μl and test times withinten seconds after application of the sample.

BACKGROUND

Electrodes are well known devices which permeate industry, and which,although often very small in size and not particularly visible, can havea significant impact on peoples' lives. Electrodes are used inelectronic instruments having many industrial, medical, and analyticalapplications. To name just a few, they include monitoring andcontrolling fluid flow, and various types of analytical methods whereinelectric current is measured to indicate the presence or concentrationof certain chemical species.

With respect to analytical methods, the need for detection andquantitative analysis of certain chemicals found within a largercomposition can be important for the chemical and manufacturingindustries, as well as biotechnology, environmental protection, andhealth care industries. Examples of substances that may be analyzedinclude liquid samples such as tap water, environmental water, andbodily fluids such as blood, plasma, urine, saliva, interstitial fluid,etc.

Many analytical techniques, sometimes referred to as electrochemicaldetection methods, make use of electrodes as a component of anelectrochemical sensor. The sensors are used in combination withelectronic apparatuses to precisely detect the presence or concentrationof a selected chemical species (analyte) within a substance sample.Techniques that allow the use of miniaturized disposable electroanalyticsample cells for precise micro-aliquote sampling, and self-contained,automatic means for measuring the analysis, can be particularly useful.

Electrochemical detection methods can include amperometric measurementtechniques, which generally involve measurement of a current flowingbetween electrodes that directly or indirectly contact a sample of amaterial containing an analyte, and studying the properties of thecurrent. The magnitude of the current can be compared to the currentproduced by the system with known samples of known composition, e.g., aknown concentration of analyte, and the quantity of analyte within thesample substance can be deduced. These types of electrochemicaldetection methods are commonly used because of their relatively highsensitivity and simplicity.

Micro-electrode arrays are structures generally having two electrodes ofvery small dimensions, typically with each electrode having a commonelement and electrode elements or micro-electrodes. If “interdigitated”the arrays are arranged in an alternating, finger-like fashion (See,e.g., U.S. Pat. No. 5,670,031). These are a sub-class ofmicro-electrodes in general. Interdigitated arrays of micro-electrodes,or IDAs, can exhibit desired performance characteristics; for example,due to their small dimensions, IDAs can exhibit excellent signal tonoise ratios.

Interdigitated arrays have been disposed on non-flexible substrates suchas silicon or glass substrates, using integrated circuitphotolithography methods. IDAs have been used on non-flexible substratesbecause IDAs have been considered to offer superior performanceproperties when used at very small dimensions, e.g., with featuredimensions in the 1-3 micrometer range. At such small dimensions, thesurface structure of a substrate (e.g., the flatness or roughness)becomes significant in the performance of the IDA. Because non-flexiblesubstrates, especially silicon, can be processed to an exceptionallysmooth, flat, surface, these have been used with IDAs.

SUMMARY OF THE INVENTION

Whereas micro-electrodes have in the past been used with non-flexiblesubstrates such as silicon, ceramic, glass, aluminum oxide, polyimide,etc., it has now been discovered that micro-electrode arrays, forexample, IDAs, can be advantageously useful when disposed on flexiblesubstrates. Moreover, such micro-electrodes, disposed on flexiblesurfaces, can be prepared using methods that involve flexible circuitphotolithography, as opposed to methods relating to integrated circuitphotolithography.

An interdigitated array of the invention, disposed on a flexiblesubstrate, can be used generally, in applications where IDAs are knownto be usefully employed. In particular embodiments of the invention, theIDAs can be used to construct electrochemical sensors, test cells, ortest strips. The sensors can be used with electronic detection systems(sometimes referred to as “test stands”) in methods of analyzing samplecompositions for analytes. Preferred embodiments of sensors can bedisposable, and can include channels or microchannels, preferably acapillary, which facilitates flow of a substance sample into thereaction chamber and in contact with the sensor.

The micro-electrode arrays of the invention can be useful when disposedonto a flexible substrate. In particular, IDAs are shown to be effectiveat dimensions relatively larger than the dimensions often used for IDAsdisposed on non-flexible substrates. Even though they can be relativelylarger than IDAs disposed on non-flexible substrates, the inventive IDAsare still able to exhibit performance properties, e.g., signal to noiseamplification benefits and steady-state assay profiles, comparable toIDAs having smaller dimensions.

Electrochemical sensors of the invention have been found to provideperformance advantages, e.g., relative to commercially availablesensors. For sensors used in glucose monitoring, compared tocommercially available sensors, the inventive sensors can exhibitimproved (shortened) processing periods, e.g., one half second tosteady-state after application of the assay potential and 5 seconds toreadout, and the ability to get an accurate and precise readout from arelatively small sample of substance, e.g., less than one microliter(μl), preferably a sample volume in the range from about 0.25 to 0.1 μl,e.g., from about 0.4 to about 0.1 μl.

The use of larger-dimensioned micro-electrode arrays also allows thesignificant advantage of fabricating arrays and sensors using relativelyless expensive and more efficient flex circuit photolithographyprocesses. These can advantageously incorporate the use of solidmaterials instead of spin-on liquid materials, e.g., one or more of asolid photoresist or a solid coverlay, instead of liquid materialstypically used in integrated circuit photolithography.

An aspect of the invention relates to micro-electrodes used incombination with a flexible substrate. The array can include a workingelectrode and a counter electrode, each including a common lead andcommonly-connected electrode elements, for example with the electrodeelements being arranged in a substantially-parallel, alternatingfashion. Preferred dimensions for micro-electrodes can be, e.g., featuresize or width of electrodes (W_(e)) in the range from 15 or 20 or 25 μm,up to about 100 μm, more preferably from greater than or about 25 or 30μm to about 50 μm. Preferred spacing between electrodes (W_(g)) can alsobe in the range from about 15 to about 50 μm, more preferably fromgreater than or about 20 or 25 μm to about 45 μm.

Another aspect of the invention relates to an electrochemical sensorcomprising an array of micro-electrodes disposed on a flexiblesubstrate. The sensor can further include a chemical coating disposed onthe array to facilitate practice of electrochemical detection methods.

Yet another aspect of the invention relates to a method of detecting ananalyte using an array of micro-electrodes of the invention, e.g., usingan electrochemical sensor comprising an interdigitated array disposedproximal to a flexible substrate. Such a method can include certain ofthe following steps. A sensor is provided which comprisesmicro-electrodes proximal to a flexible substrate, and a chemicalcoating proximal to the micro-electrodes; the coating comprises acompound reactive to produce an electroactive reaction product. Thecoating is contacted with a substance comprising an analyte, allowingthe analyte to react with chemical components of the coating to producean electroactive reaction product. Electric properties of the coatingcan be measured, and the electric properties can be correlated to theamount of electroactive reaction product, and to the amount of analyte.

Still another aspect of the invention relates to a method of preparing amicro-electrode, including the step of disposing the micro-electrodeonto a flexible substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of an interdigitated array of the invention.

FIG. 2 shows a top view of a sensor of the invention.

FIG. 3 shows a top view of a sensor of the invention.

FIG. 4 shows a side view of a sensor of the invention.

FIG. 5 shows a side view of a sensor of the invention.

FIG. 6 shows a perspective view of a dissembled sensor of the invention.

FIG. 7 shows data of assay current versus blood glucose level.

DETAILED DESCRIPTION

An embodiment of the present invention is directed to arrays ofmicro-electrodes, e.g., an interdigitated array of electrodes (sometimesreferred to as “microband” electrodes) used in combination with aflexible substrate.

An array of micro-electrodes includes two electrodes, referred to as theworking electrode and the counter electrode, electrically insulated fromone another.

Micro-electrodes, as distinguished from other electrodes generally, areunderstood in the electronic and biosensor arts. In analyzing a liquidsample using electrodes and electronic equipment and techniques, thesize and spacing of electrodes can affect whether diffusion of ananalyte through the sample to an electrode occurs by a planar ornon-planar path. Micro-electrode arrays are of a size and spacing suchthat in detecting chemical species of a solution, the species willdiffuse toward or approach an electrode of the micro-electrode array ina non-planar fashion, e.g., in a curved or hemispherical path ofdiffusion. In contrast, non-microelectrodes, i.e., “macro-electrodes,”cause diffusion of an analyte through a solute according to asubstantially planar path. It is also understood that some electrodeconfigurations can cause diffusion to take place by a mix of planar andnon-planar paths, in which case the electrodes can be considered amicro-electrode array, especially if the diffusion occurs predominantly(e.g., greater than 50%) according to a non-planar path, or if the sizeof the electrodes is less than 100 μm, e.g., less than 50 μm.

The electrodes of a micro-electrode array are positioned near each otherin an arrangement that will result in non-planar diffusion as described.The arrangement of the electrodes can be any arrangement that results insuch diffusion, with a working and a counter electrode beingsubstantially evenly spaced from each other. One electrode may bearranged into a shape or figure or outline that will produce intersticeswithin which the second electrode may be placed. For instance, oneelectrode can be arranged as an increasing radius, substantiallycircular spiral, with a continuous, long and narrow interstitial areabeing created between each successively larger revolution of electrode.The other electrode can be positioned in the interstitial area betweenrevolutions, while the electrodes remain insulated from one another. Thewidth and spacing of the electrodes can be arranged to result inmicro-electrode array performance.

According to other forms of such micro-electrode arrays, the spiral maynot be substantially circular, but could include linear, square, angled,or oblong or oval features. Or, the electrodes could be arranged in anyother geometric form whereby the electrodes are placed adjacent to eachother and within the other's respective interstitial area, e.g., byfollowing a similar path separated by a substantially uniform gap.

In one particular embodiment, the micro-electrode can be arranged intoan interdigitated array, meaning that at least a portion of electrodeelements of the working electrode are placed substantially parallel toand in alternating succession with at least a portion of the electrodeelements of the counter electrode, e.g., in an alternating,“finger-like” pattern. Such interdigitated micro-electrode arraysinclude electrode elements (sometimes referred to as “fingers”) and acommon element (“contact strip”) which commonly connects the electrodeelements.

The components of the electrodes may be made of any conductive material,including those known and conventionally used as electrode materials,particularly including materials known in the flexible circuit andphotolithography arts. These can include, for example, carbon, noblemetals such as: gold, platinum, palladium, alloys of these metals,potential-forming (conductive) metal oxides and metal salts, as well asothers.

The electrodes and their components can be of dimensions, meaning thewidth of the electrode components as well as the separation betweencomponents, that can provide an array with useful properties, e.g.,useful or advantageous capabilities with respect to contacting asubstance or measuring electrical properties. Advantageously,interdigitated arrays can be prepared at dimensions that allow forcontact with and measurement of electrical properties of a relativelysmall sample of a substance.

In preferred embodiments of the invention, each electrode element canindependently have a width (W_(e)) in the range from greater than 15micrometers (μm) to about 50 μm, with the range from greater than orabout 20 or 25 μm to about 40 μm being particularly preferred. Theseparation between electrode components (W_(g)), especially theseparation between alternating electrode elements, can also preferablybe in the range between about 15 micrometers and about 50 μm, with therange from greater than or about 20 or 25 μm to about 40 μm beingparticularly preferred. The total area of an electrode (meaning the areaof the fingers but not the common element) can be chosen depending onthese dimensions, on the use intended for the electrode, on the desiredcurrent level intended to pass through the electrode, and on the desirednumber of electrode elements. An exemplary area of an electrode having10 electrode elements can be in the range from about 0.1 to about 0.5square millimeters, (for example 10 electrode fingers having dimensionsof 50 μm by 1 mm), e.g., from about 0.2 to 0.3.

The thickness of the electrode components can be sufficient to support adesired electric current. Exemplary thicknesses can be in the range fromabout 30 to 200 nanometers (nm), with a preferred thickness being about100 nm.

The electrodes can independently have a number of interdigitatedelectrode elements sufficient to provide utility, e.g., allowing contactwith a substance to measure its electrical behavior. Conventionally, thearray can have substantially the same number (equal, plus or minus one)of electrode elements in the working electrode as are in the counterelectrode, allowing the electrode elements to be paired next to eachother in an alternating sequence. In some preferred embodiments of thearray, such as in some of the applications described below forelectrochemical sensors, each electrode of an array may typically havefrom about 4 to about 30 electrode elements.

FIG. 1 illustrates an embodiment of an array of the invention. Workingelectrode 2 and counter electrode 4 are arranged as an interdigitatedarray on flexible substrate 10. (The figure is not to scale and itsdimensions, as well as the dimensions of the other figures, should notbe construed to limit the invention). The working and counter electrodesinclude common strips 6 a and 6 b, respectively, which can be connectedto electrically conductive means (e.g., “connectors,” “pads,” or“leads,” etc.) for connecting the electrodes to an external circuit. Inthe illustrated example, the working electrode includes electrodeelements 8 a connected to common strip 6 a, and the counter electrodeincludes electrode elements 8 b connected to common strip 6 b.

According to the invention, the interdigitated array is disposedproximal to, e.g., on, a flexible substrate. To act as a flexiblesubstrate, a material must be flexible and also insulating, and istypically relatively thin. The substrate should be capable of adheringcomponents of an IDA, or additional components of a sensor, to itssurface. Such thin, insulative, flexible substrates are known in the artof flexible circuits and flex circuit photolithography. “Flexiblesubstrates” according to the present disclosure can be contrasted tonon-flexible substrates used in integrated circuit (IC) photolithographybut not in flexible circuit photolithography. Examples of non-flexiblesubstrates used in IC photolithography include silicon, aluminum oxide,and other ceramics. These non-flexible substrates are chosen to beprocessable to a very flat surface. Typical flexible substrates for usein the invention are constructed of thin plastic materials, e.g.,polyester, especially high temperature polyester materials; polyethylenenaphthalate (PEN); and polyimide, or mixtures of two or more of these.Polyimides are available commercially, for example under the trade nameKapton®, from I.E. duPont de Nemours and Company of Wilmington, Del.(duPont). Polyethylene naphthalate is commercially available asKaladex®, also from duPont. A particularly preferred flexible substrateis 7 mil thick Kaladex® film.

Interdigitated arrays of the invention can be used in applicationsgenerally known to incorporate electrodes, especially applications knownto involve interdigitated arrays of electrodes. Various applications areknown in the arts of electronics and electrochemistry, includingapplications relating to process and flow monitoring or control, andchemical analytical methods. The arrays may be particularly useful as acomponent of an electrochemical sensor, where there is added value,benefit, or cost efficiency, to the use of a flexible substrate, orwhere there is value, benefit, or cost efficiency in having aninterdigitated array of dimensions relatively larger than the dimensionsof interdigitated arrays conventionally disposed on non-flexiblesubstrates.

An interdigitated array of the invention can, for example, be includedin an electrochemical sensor (sometimes referred to as a “biosensor” orsimply “sensor”) used in electrochemical detection methods.Electrochemical detection methods operate on principles of electricityand chemistry, or electrochemistry, e.g., on principles of relating themagnitude of a current flowing through a substance, the resistance of asubstance, or a voltage across the substance given a known current, tothe presence of a chemical species within the substance. Some of thesemethods can be referred to as potentiometric, chronoamperometric, orimpedance, depending on how they are practiced, e.g., whether potentialdifference or electric current is controlled or measured. The methodsand sensors, including sensors of the invention, can measure currentflowing through a substance due directly or indirectly to the presenceof a particular chemical compound (e.g., an analyte or an electroactivecompound), such as a compound within blood, serum, interstitial fluid,or another bodily fluid, e.g., to identify levels of glucose, bloodurea, nitrogen, cholesterol, lactate, and the like. Adaptations of someelectrochemical methods and electrochemical sensors, and features oftheir construction, electronics, and electrochemical operations, aredescribed, for example, in U.S. Pat. Nos. 5,698,083, 5,670,031,5,128,015, and 4,999,582, each of which is incorporated herein byreference.

Oftentimes, a compound of interest (analyte) in a substance is notdetected directly but indirectly, by first reacting the analyte withanother chemical or set of chemicals proximal to or in contact with anIDA. The reaction produces an electroactive reaction product that iselectrochemically detectable and quantifiable by applying a potentialdifference between the counter and working electrodes and measuring themagnitude of the current produced. This allows measurement of the amountof electroactive reaction product generated by the first reaction, andcorrelation of that measurement to the amount of analyte in the samplesubstance.

An example of such a method involves the catalytic use of an enzyme, andis sometimes referred to as enzymatic amperometry. These methods can usean interdigitated array of electrodes coated with a chemical coatingthat contains a chemical compound reactive to produce an electroactivereaction product. (The chemical compound reactive to produce anelectroactive reaction product is sometimes referred to herein as a“mediator.”) Upon contacting the coating with a sample that contains ananalyte, analyte reacts with chemical compounds of the coating togenerate electroactive reaction product. This electroactive reactionproduct can be electronically detected, measured, or quantified, byapplying a potential difference between the electrodes and measuring thecurrent generated by the electrooxidation of the mediator at the workingelectrode. By calibrating the system's behavior using known substancesand concentrations, the electrical behavior of the system in thepresence of a sample substance of unknown composition can be determinedby comparison to the calibration data.

The sensor of the invention may be used in amperometric applications,e.g., enzymatic amperometric applications, if disposed on the array is acoating of useful chemistry, including e.g., an enzyme and a mediator.When a sample containing an analyte is contracted with the coating, theanalyte, enzyme, and the mediator participate in a reaction, wherein themediator is either reduced (receives at least one electron) or isoxidized (donates at least one electron). Usually, in this reaction, theanalyte is oxidized and the mediator is reduced. After this reaction iscomplete, an electrical potential difference can be applied between theelectrodes. The amount of reducible species and the applied potentialdifference must be sufficient to cause diffusion-limitedelectrooxidation of the reduced form of the mediator at the surface ofthe working electrode. The IDA electrode configuration of the sensorplaces the working electrode fingers in close proximity to counterelectrode fingers. Mediator electrooxidized at the working electrode cantherefore diffuse rapidly to the adjacent counter electrode via radialdiffusion where it is once again reduced. Likewise, oxidized mediatorreduced at the counter electrode can migrate to the working electrodefor electrooxidation to the oxidized form. This migration between thefingers produces a constant or “steady state” current between theelectrodes. After a short time delay, this steady state current ismeasured and correlated to the amount of analyte in the sample.

The chemistries of the first and second reactions can be of any natureeffective to produce the electroactive reaction product of the firstreaction, to detect or quantify the electroactive reaction productduring the second reaction, and to allow correlation of the amount ofelectroactive reaction product with the presence or concentration ofanalyte in the original sample.

In general, a typical first reaction can be an oxidation/reductionsequence, preferably occurring without the need for a chemical potentialacross the electrodes. It can be desirable for this reaction to favormaximum, preferably complete conversion of the analyte, and to proceedas quickly as possible. Often this reaction is catalyzed, e.g.,enzymatically. Such reaction schemes and their application to enzymaticamperometry are known. See, e.g., U.S. Pat. No. 5,128,015; EuropeanPatent Specification EP 0 406 304 B1; and Aoki, Koichi, QuantitativeAnalysis of Reversible Diffusion-Controlled Currents of Redox SolubleSpecies at Interdigitated Array Electrodes Under Steady-StateConditions, J. Electroanal. Chem. 256 (1988) 269-282. An example of auseful reaction scheme can be the reaction of a component of a bodilyfluid, e.g., glucose, with an enzyme and a cofactor, in the presence ofa mediator, e.g., an oxidizer, to produce an electroactive reactionproduct.

The chemistry of a first reaction scheme of any chosen electrochemicaldetection method can be chosen in light of various chemical factorsrelating to the system, including the identity of the analyte and of thesample substance. Even then, for a given analyte or substance, variousdifferent reactive components may be useful in terms of a catalyst(often, a variety of enzymes will be useful), co-reactants (e.g., avariety of mediators may be useful), and cofactors (if needed, a varietymay be useful). Many such reaction schemes and their reactive componentsand reaction products are known, and examples of a few different enzymesinclude those listed in Table 1.

Redox Mediator Analyte Enzymes (Oxidized Form) Glucose Glucosedehydrogenase Ferricyanide, osmium and Diaphorase (III)-(bipyridyl)-2-imidazolyl-chloride, Meldola blue, [Ru(NH₃)₅MeIm] Cl₃ [OS(III)(NH₃)₅pyz]₂(SO₄)₃, NITROSO analine derivatives Glucose Glucose oxidase(see above) Cholesterol Cholesterol esterase and (see glucose)Cholesterol oxidase HDL Cholesterol esterease (see glucose) Cholesteroland Cholesterol oxidase Triglycerides Lipoprotein lipase, (see glucose)Glycerol kinase, Glycerol-3-phosphate oxidase Triglycerides Lipoproteinlipase, (see glucose) Glycerol kinase, Glycerol-3-phosphatedehydrogenase and Diaphorase Lactate Lactate oxidase (see glucose)Lactate Lactate dehydrogenase (see glucose) and Diaphorase LactateDiaphorase (see glucose) Dehydrogenase Pyruvate Pyruvate oxidase (seeglucose) Alcohol Alcohol oxidase (see glucose) Alcohol Alcoholdehydrogenase (see glucose) and Diaphorase Uric acid Uricase (seeglucose) 3- 3-Hydroxybutyrate (see glucose) Hydroxybutric dehydrogenaseand acid (ketone Diaphorase bodies)

A mediator can be any chemical species (generally electroactive), whichcan participate in a reaction scheme involving an enzyme, an analyte,and optionally a cofactor (and reaction products thereof), to produce adetectable electroactive reaction product. Typically, participation ofthe mediator in this reaction involves a change in its oxidation state(e.g., a reduction), upon interaction with any one of the analyte, theenzyme, or a cofactor, or a species that is a reaction product of one ofthese (e.g., a cofactor reacted to a different oxidation state). Avariety of mediators exhibit suitable electrochemical behavior. Amediator can preferably also be stable in its oxidized form; mayoptionally exhibit reversible redox electrochemistry; can preferablyexhibit good solubility in aqueous solutions; and preferably reactsrapidly to produce an electroactive reaction product. Examples ofsuitable mediators include benzoquinone, medula blue, other transitionmetal complexes, potassium ferricyanide, and nitrosoanalines, see U.S.Pat. No. 5,286,362. See also Table 1.

To describe an example of an oxidation/reduction reaction scheme that isknown to be useful for detecting glucose in human blood, a samplecontaining glucose can react with an enzyme (e.g.,Glucose-Dye-Oxidoreductase (Gluc-Dor)) and optionally a cofactor, (e.g.,pyrrolo-quinoline-quinone), in the presence a redox mediator (e.g.,benzoquinone, ferricyanide, or nitrosoanaline derivatives), to producethe oxidized form of the analyte, gluconolactone, and the reduced formof the redox mediator. See U.S. Pat. No. 5,128,015. Other examples ofreaction schemes are known, and are typically used in methods designedto detect a specific analyte, e.g., cholesterol, urea, etc.

After the reaction is complete, a power source (e.g., battery) applies apotential difference between the electrodes. When the potentialdifference is applied, the amount of oxidized form of the redox mediatorat the counter electrode and the potential difference must be sufficientto cause diffusion-limited electrooxidation of the reduced form of theredox mediator at the working electrode surface. In this embodiment, theclose proximity of the counter and working electrode fingers in the IDAelectrode configuration aids in the fast radial diffusion of the reducedand oxidized redox mediator between the electrodes. Recycling of themediator between the electrodes and their subsequent oxidation andreduction on the electrodes generates a constant or “steady state” assaycurrent. This steady state assay current is measured by a currentmeasuring meter.

The measured current may be accurately correlated to the concentrationof analyte in the sample when the following requirements are satisfied:

1) the rate of oxidation of the reduced form of the redox mediator isgoverned by the rate of diffusion of the reduced form of the redoxmediator to the surface of the working electrode; and

2) the current produced is limited by the oxidation of the reduced formof the redox mediator at the surface of the working electrode.

In the preferred embodiment, these requirements are satisfied byemploying a readily reversible mediator and by using a mixture ofamounts of mediator and other components of the chemical layer to ensurethat the current produced during diffusion limited electrooxidation islimited by the oxidation of the reduced form of the mediator at theworking electrode surface. For current produced during electrooxidationto be limited by the oxidation of the reduced form of the mediator atthe working electrode surface, the amount of reducible species at thesurface of the counter electrode must always exceed the amount of thereduced form of the redox mediator at the surface of the workingelectrode.

An example of a reaction scheme relates to the detection of glucoseusing ferricyanide and Glucose-Dye-Oxidoreductase (Glur-Dor). Theelectroactive reaction product of the enzymatic reaction between glucoseand the enzyme is the reduced mediator, ferrocyanide. The ferrocyanideis electrooxidized at the working electrode back to ferricyanide. Onemole of oxidized redox mediator is reduced at the counter electrode forevery mole of reduced redox mediator oxidized at the working electrode.Ferricyanide electrooxidized at the working electrode, diffuses to thecounter electrode, and the ferrocyanide produced at the counterelectrode can rapidly diffuse to the working electrode where it is againoxidized. A “quasi-steady state” concentration gradient is establishedbetween the counter and working electrode pairs resulting in generationof a constant quasi-steady state current at the working electrode.

The magnitude of the current, preferably as measured at aquasi-steady-state condition, can be correlated to the amount ofelectroactive reaction product present in the coating, and consequently,to the amount of analyte in the sample.

The chemical coating should allow diffusion of analyte into the coating,followed by reactions as described. The coating can include materialswhich can contain the reactive chemical components, which allow reactionbetween the components to product an electroactive reaction product,which allow necessary diffusion of chemical components, and which cansupport a current passing through the coating based on the concentrationof electroactive reaction product. Typically, the coating can be made upof a binder that contains a set of chemicals which react to produce anelectroactive reaction product. The chemicals generally include amediator and necessary enzymes and cofactors. Such a coating can alsocontain a variety of additional components to make the coating operativeand suitable for processing, including specific components listed aboveas well as surfactants, film formers, adhesive agents, thickeners,detergents, and other ingredients and additives that will be understoodby an artisan skilled in the electrochemical sensor art.

The binder can provide integrity of the coating while allowing diffusionof the different components of the reaction scheme, reaction between thereactive components, and movement of reactive components and productssufficient to produce a quasi-steady-state concentration gradient ofmediator and electroactive reaction product and thereby establish astable or quasi-steady-state current between the electrode pairs.Exemplary binders can include gelatin, carrageenan, methylcellulose,polyvinyl alcohol, polyvinylpyrrolidone, alignate, polyethylene oxide,etc.

A sensor according to the invention can be understood to include amicro-electrode disposed on a flexible substrate, optionally including achemical coating, and further including any immediate appurtenancenecessary to use the sensor in an electronic system or apparatus (e.g.,test stand) designed, for example, for use in an electrochemicaldetection method. A sensor can include the interdigitated array disposedon a flexible substrate, with additional components to independentlyconnect each of the separate electrodes to a different voltage, e.g.,electrical connectors, leads, or pads. In some circumstances, the sensormay include a reference electrode provided on the same or a differentsubstrate and electrically insulated from the interdigitated array. Thesensor may also include components to direct flow of a sample substanceinto contact with the IDA, e.g., a vessel, channel, microchannel, orcapillary. A particularly preferred embodiment of the sensor includes amicrochannel or capillary, most preferably a capillary, which directsflow of a sample substance into the reaction chamber and over the IDA(e.g., a coated IDA).

A capillary can be included in a sensor to facilitate analysis of asmall volume of a sample substance by precisely directing the flow of avolume of sample over the IDA, preferably in a short period of time.Analysis of relatively small volumes of a sample substance can beaccomplished, at least in part, due to the signal amplification featuresof the IDA.

Preferred dimensions of a capillary for what can be referred to as a“low volume sensor configuration,” can be in the range of 0.025 mm to0.2 mm (depth), preferably about 0.125 mm (depth), ×1 mm (width)×3 mm(length), resulting in a capillary chamber requiring a relatively smallvolume of sample, e.g., less than 400 nanoliters (nL). The volume of thechamber can preferably be such that a low volume sample of a substancecan be directed into or through the chamber for analysis. Chambervolumes will vary depending on the type of analyte being studied, andeven its concentration of an analyte. (Blood samples of differenthematocrits will dispense differently into a capillary.) Exemplarychamber volumes can be in the range from about 100 to 300 nanoliters forglucose analysis in interstitial fluid, and from about 250 to 400microliters for glucose analysis applications in the whole blood. In themost preferred embodiments of the sensor, including a capillary, thecapillary may have a vent to facilitate flow of a sample substance intothe capillary chamber by equalizing pressure between the interior andexterior of the chamber.

The sensor of the invention can include these and other features, and,especially if an embodiment is disposable, can be referred to as a “teststrip” or a “test cell.” The term “disposable” refers to sensorsdesigned or sold for a single use, after which they are to be discardedor otherwise stored for later disposal.

Capillaries may be fabricated as a component of a sensor, usingphotolithographic methods, e.g., as described infra.

An example of a sensor construction is shown in FIG. 2, according to thepreferred embodiment. The figure shows sensor 20, including aninterdigitated array of electrodes 22 disposed on flexible substrate 24.The electrodes are connected to electrically-conductive connectors 26which include portions 28 that can be identified as pads, located on thesurface of the flexible substrate, where they are available to becontacted to an external electronic circuit such as a testing apparatus.The connectors also include connector portions 30, which connectelectrode elements at the array to the pads, and which may typically becovered by an insulating layer. FIG. 2 a shows a close-up of array 22,showing that electrodes attached to each of connectors 26 are arrangedin an inter digitated fashion (as shown in FIG. 1).

FIG. 3 shows different details of a sensor of the invention. FIG. 3shows sensor 20 comprising flexible substrate 24, an array ofinterdigitated electrodes 22, and connectors and pads. Non-conductivelayer 32 is disposed over the substrate and connector portions 30 of theconnectors 26, over portions of the array 22, and not over a rectangularcapillary portion including some of the substrate and an intersection ofarray 22; this rectangular portion defines capillary chamber 34. (Achemical coating, not shown in this figure, is preferably disposed overthe array, within the capillary chamber.) Foil 36 covers a rectangularportion of the sensor, including portions of the non-conductive layer32, and a portion of capillary chamber 34, except for air vent 38. Thisembodiment is shown from one side in FIG. 4, and from another side inFIG. 5. FIG. 5 specifically illustrates substrate 24, array 22,non-conductive layer 32, which defines chamber 34, and foil 36. FIG. 5additionally includes coating 40 disposed on array 22, within thecapillary.

FIG. 6 illustrates an exploded view of a sensor of the invention. Thesensor 20 includes flexible substrate 24; a conductive film 40 patternedwith an interdigitated array of electrodes 22 and connectors 26 whichinclude pad portions 28 and connecting portions 30, an insulatingmaterial 32 which defines the depth and dimensions of capillary chamber34, a chemical coating 40 disposed in the capillary chamber 34, and topfoil 36 coated with a hydrophilic adhesive layer 42.

The array of the invention, in various embodiments such as a sensor, canbe used in electrochemical detection methods, including those using theprinciples and specific methods described above, and others. Suchmethods employ the array disposed on a flexible substrate, preferablyfurther including a chemical coating contacting the array.

Upon contacting the coating with a sample containing analyte, analytegenerally diffuses into the coating at a rate dependant on factors suchas the chemical composition of the coating and the chemical identity ofthe analyte. Generally, the chemical coating will be at least partlysolubilized or hydrated by the sample substance. For a method to providethe quickest read time (the time following contact with a substancesample, when a reading of the concentration of analyte in the substanceis available), it is desirable that the analyte diffuse quickly into thecoating, and thereafter quickly and completely react to produce anelectroactive reaction product. The period during which this occurs canbe reduced by operating on a relatively small volume of sample, and byusing a sensor having a relatively small amount of chemical coating tobe solubilized or hydrated.

The time from when the substance containing the analyte is contactedwith the chemical coating until an assay potential is applied to thearray, and during which the analyte diffuses into the coating and reactsto produce an electroactive reaction product, can be referred to as the“delay period”. This period can be any amount of time necessary for theabove occurrences to transpire, is preferably minimized, and in someembodiments can be less than about 10 seconds, preferably in the rangefrom about 2 to 6 seconds.

After a delay period, the electric properties of the coating can bemeasured. By chronoamperometric methods, or by potentiometric methods,as will be appreciated by the skilled artisan, either the current or theapplied potential can be controlled, and any of the related current,resistance, or voltage can be measured and correlated to amounts ofelectroactive reaction product and analyte. The magnitude of thecurrent, or alternatively potential difference or the resistance of thechemical coating, can be measured using an external circuit connected tothe sensor electrodes.

As an example, according to chronoamperometric methods, a potential(“assay potential”) can be applied across the electrodes, inducing acurrent (“assay current”) to flow through the coating. The potentialshould be enough to cause reduction or oxidation of the redox productsformed in the first step of a binary reaction scheme (e.g., as describedabove), but should not be sufficient to cause other electrochemicalreactions or to otherwise cause significant current to flow through thecoating. The assay potential can be chosen depending on the redoxmediator chosen, factors relating to the electrochemical detectionmethod, the electrochemical system and reaction scheme, and the generalcapabilities of the sensor. A typical potential can be in the range of afew to several hundred millivolts, e.g., from about 100 to 500,preferably 200 to 400 millivolts.

A measured current can initially exhibit a spike to a relativelyelevated level, and can then descend to a steady-state current based ona quasi-steady-state concentration gradients and a recycle reaction loopof the mediator and electroactive reaction product. Preferably, themagnitude of the current can be measured at a time when current flowingthrough this system has approached a plateau, based onquasi-steady-state concentration gradients. The period of time startingwith application of the assay potential and lasting to the plateau ornear-steady-state current can be referred to as the “assay period.”Steady-state assay currents can occur within various such time periods,depending upon the reaction scheme, the chemistries of its components,etc. In the practice of the invention, assay periods of less than oneminute are preferred, e.g., less than 30 seconds, and assay periods ofeven shorter duration, less than 10 seconds, are most preferred. Theassay profile (the profile of the assay current over time) can be tosome extent controlled by changing the spacing between electrodeelements in the array; increased spacing between electrode elements canresult in a longer time interval between assay potential application andformation of the steady state assay currents.

Assay currents exhibited by exemplary sensors of the invention can beany current that will function in an electrochemical detection method.For the sensors of the invention, any useful current can be used,preferably with a range between a lower end in the nanoamp range (e.g.,between 20 to 25 nanoamps) up to the microamp range e.g., 100 microamps,being an exemplary working range, e.g., at the steady state currentplateau. Typical steady state assay currents can be in the range frombelow one microamp up to around 100 microamp, preferably from about 0.5to about 25 microamps. In an embodiment of the invention useful fordetecting glucose content of a blood sample, the current response(steady state assay current) in this range has been found to be linearwith respect to the concentration of glucose in the sample, particularlyfor glucose concentrations in the range from about 0 to 600 milligramsper deciliter (mg/dL).

Sensors of the invention may be used in cooperation with electronic orcomputerized systems and apparatuses, and in combination with methodsfor identifying analytes and measuring concentration of analytes withina substance sample. For example, a sensor can be used with a VXI orBiopotentiostat test stand built from components purchased from NationalInstrument Corp., Austin, Tex. In this context, the method of theinvention can be practiced with a delay period of around 3 seconds, anassay potential of about 300 millivolts, and an assay period which,although variable, can preferably be in the range from about 1.5 to 2seconds after applying the assay potential.

The sensors can be used in such a method to detect and quantify theconcentration of an analyte within a sample substance. The analyte canbe chosen from various chemical compounds present within any of a largevariety of substances, generally fluids. Examples of analytes includeglucose, cholesterol, urea, and the like. Examples of substancescontaining the analyte include bodily fluids such as blood, urine, andinterstitial fluid; water such as environmental water, ground water,waste water, etc.

In some embodiments of the invention, analytes can be detected at verylow concentrations, for example glucose can be measured atconcentrations as low as 0.5 mg/dL (5 ppm) in blood using ferricyanideas the mediator.

The use of an array or sensor of the invention offers certain practicaladvantages. For instance, a flexible substrate can be used incombination with relatively larger-dimensioned electrodes, includingelectrode components of increased size (e.g., width) as well asincreased spacing between them. Lower sample volumes can independentlydecrease the time of the delay period. A shorter delay period incombination with an expedited formation of a quasi-steady-state regionof the assay current produces a quicker read time. In the practice ofthe invention, read times of less than 10 seconds have been achieved,with a read times in the range from about 4 and 5 seconds beingpreferred.

Test cells and test strips according to the invention allow forcontrolled volumes of blood to be analyzed without pre-measuring.Insertion of the test cell into an electronic or computer-controlledapparatus (referred to generally as a test stand) permits automaticfunctioning and timing of the reaction and analysis of the sample. Thisallows for patient self-testing with a very high degree of precision andaccuracy. The method, the sensor or test cell, and the apparatus, aredesigned to provide self-monitoring by a patient of important bodilyfluids, e.g., blood glucose levels. The sensor is used to control thesample volume and reaction media, to provide precise, accurate, andreproducible analysis. Preferably, disposable test strips or test cellscan be used in combination with a portable electrochemical testingmeter.

The preferred embodiment of the present invention uses a micro-electrodearray consisting of interdigitated micro-band electrodes as describedabove. Although this arrangement leads to the aforementioned re-cyclingof redox products between narrowly separated working and counterelectrodes, this is not a strict requirement for successful practice ofthe invention. An alternative embodiment is the provision of an array ofmore general micro-electrodes to act as the working electrode structure.These may be micro-bands that are not interdigitated with the counterelectrode, or micro-disks, also not closely spaced with the counterelectrode. In this case the width or diameter of the working electrodebands or disks should be of such a dimension as to allow for some degreeof radial or spherical diffusion to the working electrode surfaces.Typically, this dimension should be in the range of 5 to 50 μm, and mostpreferably 10 to 50 μm for the case of aqueous systems such asencountered with a sensor used for the assay of biological fluids. Inboth cases the counter electrode is provided at a distance from theworking array that is generally larger than the smallest dimension ofthe working electrodes.

In these embodiments, specific recycling of redox species between theworking and counter electrodes is not observed in the same way as inother described embodiments, and assay current magnitudes areconsequently reduced. Nevertheless, the effect of radial or sphericaldiffusion to working micro-electrode structures can still be observed ascurrent densities that are greater than that predicted from lineardiffusion alone. Although reduced in magnitude, and not approachingquasi-steady-state as displayed by the preferred embodiments, it isstill possible to measure dose responses to the analyte in question(e.g. glucose) when the same reagent as described above is disposed onthe micro-electrode array.

Micro-electrode arrays of the invention can be disposed onto a flexiblesubstrate using various methods useful for disposing electroniccomponents onto substrates, especially flexible substrates. A variety ofsuch methods are generally known for fabrication of different types ofcircuitry, and include specific techniques of dry-coating, lamination,spin-coating, etching, and laser ablation. One or more of the followinggeneralized methods may be specifically useful to prepare microelectrodearrays according to the invention.

One method of preparing a micro-electrode array as described herein,e.g., an IDA, is by the use of laser ablation techniques. Examples ofthe use of these techniques in preparing electrodes for biosensors aredescribed in U.S. patent application Ser. No. 09/866,030, “Biosensorswith Laser Ablation Electrodes with a Continuous Coverlay Channel” filedMay 25, 2001, and in U.S. patent application Ser. No. 09/411,940,entitled “Laser Defined Features for Patterned Laminates and Electrode,”filed Oct. 4, 1999, both disclosures incorporated herein by reference.

In general, laser ablative techniques use a laser to cut or mold amaterial. According to the invention, micro-electrodes can be preparedusing ablative techniques, e.g., by ablating a multi-layer compositionthat includes an insulating material and a conductive material, e.g., ametallic laminate of a metal layer coated on or laminated to aninsulating material. The metallic layer may contain pure metals oralloys, or other materials which are metallic conductors. Examplesinclude aluminum, carbon (such as graphite), cobalt, copper, gallium,gold, indium, iridium, iron, lead, magnesium, mercury (as an amalgam),nickel, niobium, osmium, palladium, platinum, rhenium, rhodium,selenium, silicon (such as highly doped polycrystalline silicon),silver, tantalum, tin, titanium, tungsten, uranium, vanadium, zinc,zirconium, mixtures thereof, and alloys or metallic compounds of theseelements. Preferably, the metallic layer includes gold, platinum,palladium, iridium, or alloys of these metals, since such noble metalsand their alloys are unreactive in biological systems. The metalliclayer may be any thickness but preferably is 10 nm to 80 nm, morepreferably 20 nm to 50 nm.

In the laser ablation process, the metallic layer may be ablated into apattern of micro-electrodes. The patterned layer may additionally becoated or plated with additional metal layers. For example, the metalliclayer may be copper, which is then ablated with a laser, into anelectrode pattern. The copper may be plated with a titanium/tungstenlayer, and then a gold layer, to form desired micro-electrodes.Preferably, however, in some embodiments, only a single layer of gold isused. One example of a useful metallic laminate is a polyester or otherflexible substrate such as a Kaladex film, coated with a layer of gold,preferably about 5 mils in thickness.

The conductive material is ablated with the laser to leave amicro-electrode array. Any laser system capable of ablation of theconductive material will be useful. Such laser systems are well knownand commercially available. Examples include excimer lasers, with apattern of ablation controlled by lenses, mirrors, or masks. A specificexample of such a system is the LPX-400, LPX-300, or LPX-200, both fromLPKF LASER ELECTRONIC, GMBH of Garbsen, Germany.

One specific example of a process for laser ablation is as follows.Sheets of sensor traces are fabricated in a MicrolineLaser 200-4 lasersystem (from LPKF). The system chamber includes a vacuum platen atop ofa LPKF-HS precision positioning X,Y table, laser mirrors and optics, anda quartz/chromium photomask (International Phototool Company, ColoradoSprings, Colo.) with the sensor components subdivided into rectangularfields on the mask. Photomask positioning, X,Y table movement and laserenergy are computer controlled. Sheets of metal laminate 22 cm×22 cm indimension are placed into the chamber onto the vacuum table. The tablemoves to the starting position and the Kr/F excimer laser (248 nm) isfocused through the first field of the photomask onto the metallaminate. Laser light passing through the clear areas of the photomaskfield ablates the metal from the metal laminate. Chromium coated areasof the photomask block the laser light and prevent ablation in thoseareas, resulting in a metallized sensor structure on the laminate filmsurface. The complete structure of the sensor traces may requireadditional ablation steps through various fields on the photomask.

Another method of preparing the described micro-electrode array is theuse of flex circuit photolithography. Flex circuit photolithographymethods are well known. Two general methods of fabricating flexiblecircuits include the “additive” method and the “subtractive” method.With the additive method, an IDA and associated circuitry can be builtup on top of a non-conductive flexible substrate. With the subtractivemethod, a non-conductive flexible substrate can be laminated with aconductive material (e.g., a copper foil) and conductive material ispatterned using conventional photolithographic and wet chemical etchingtechniques. Some conventional processing steps include cleaning asubstrate or intermediate; depositing conductive materials onto asubstrate, e.g., by vapor deposition, electrodeposition, or vacuumplasma sputtering; depositing non-conductive or processing materialsonto a substrate such as a photoresist material; masking and developinga photoresist material in a pattern defining an electrode; and removingexcess developed or non-developed materials such as photoresistmaterials or conductive materials, to leave behind a desired arrangementof electrically conductive and insulating materials.

According to one series of steps in flex circuit photolithography, asubstrate is prepared by cleaning, and a conductive material can beapplied as a film to the substrate. Preferred thicknesses of aconductive layer (e.g., a gold conductive layer) can be in the rangefrom about 500 to 1000 angstroms. It may be desirable to include a seedlayer such as titanium or chromium between the conductive layer and thesubstrate, to improve adhesion. A preferred conductive material can begold, and a preferred method of application can be sputtering, which hasbeen found to provide very good adhesion.

A photoresist material can be applied to the conductive layer. Suchphotoresist materials are commercially known and available, with oneexample being Riston® CM206, from duPont. The thickness of thephotoresist can be chosen to advantageously affect the resolution of thefeature sizes of the electrode components. Improved resolution generallyprovides for better quality arrays, with fewer failures. There has beenfound a 1:1 relationship between the resolution of the smallest featuresize achievable, and the thickness of the dry film photoresist, withthinner photoresist films providing better resolution (a thickness ofabout 0.6 mils generally allows a feature spacing or width of 0.6 mils).Riston® CM206, in the form of a 0.6 mil thick roll of film, can be apreferred photoresist because it can be capable of resolving features,i.e., lines and spaces, on a lower micron scale, e.g., in the range of0.6 mils (15 microns) or lower. A photoresist layer often requiresheating. Riston® CM206 does not require prebaking. The material is a dryfilm photoresist and is applied to the gold substrate using a heatedlaminated roller system. Once laminated, the material is ready forprocessing (exposure to UV light, and development).

The laminated film can be cut to a convenient size, e.g., one foot byone foot, and a pattern defining a micro-electrode array can be cured orcrosslinked. This can generally be accomplished by conventional methods,e.g., using a mask pattern and exposing the array pattern to ultravioletlight, crosslinking the photoresist in the pattern of the array.Unexposed, uncrosslinked, photoresist can be developed away using adeveloping agent, which will typically be particular to the photoresistcomposition (e.g., lithium carbonate is one developing agent; see themanufacturer's instructions). At the end of this step, the substratewill have an undisturbed layer of the conductive material coatedthereon, with a photoresist design defining the pattern of the arraylaid out on the conductive layer. This allows for unprotected conductivematerial to be etched away using an etchant (e.g., KI/I₂), to producethe IDA pattern in the conductive material. The remaining photoresistcan then be removed.

Once an array is prepared, e.g., by laser ablative methods, usinglaminated dry photoresist, spin coating, etching, or other techniques,further processing of the micro-electrode array can be used toincorporate the array into a useful electronic device such as abiosensor. Preferably, additional materials can be disposed onto thearray to form, for example, a spacer or insulating layer, optionallyincluding a well or a microchannel or capillary. A well refers to aspace over an array that defines the array. A microchannel or capillarymore specifically refers to a space or channel that is defined over thearray to allow the flow of a fluid over the array. The material used todefine the microchannel or capillary can be any of a variety ofmaterials useful insulating or spacing materials, sometimes referred toas “coverlay” materials, as well as other material useful for processingwith the described fabrication methods. An example is Pyralux coverlay,and similar materials would also be useful.

Methods useful to place a microchannel or capillary onto the arrayinclude methods of mechanical lamination and mechanical removal ofmaterial to form a channel or capillary. One method would include afirst step of mechanically “punching” (e.g., die punching) the coverlaymaterial to cut away one or multiple portions of the material in theform of wells or channels, and then laminating the material to one or anumber of sensors such that the channel is present over the array.Another method includes those types of methods generally referred to as“kiss die cutting” or “kiss cutting,” which may be used to cut a well orchannel in a coverlay layer, and then the coverlay material may belaminated onto the substrate with the well or channel over the array.

A different example that includes a die punching method is as follows. Aspacer foil was prepared by coating an adhesive, Fastbond™ 30-NF ContactAdhesive to a wet thickness of 25 μm onto a 5 mil polyester film such asthat sold under the trademark Melinex® S (DuPont Polyester films,Wilmington Del.) using a wire bar coater from Thomas Scientific ofSwedesboro, N.J. The coated top foil was dried for 2 minutes at 50 C. ina horizontal air flow oven. The dried adhesive on the sheet was coveredwith either silicon or teflon release liner. Capillary channels andelectrode contact well patterns were kiss cut into the sheet using anAristomat 1310 digital die cutting system (Aristo Graphic Systeme GmbH &Co., Hamburg Germany). The spacer sheet can then be registered andlaminated to an ablated sheet of sensor traces, as described above.Channels and electrode contact wells can also be produced using diepunching processes in a similar fashion.

Another specific method by which to dispose a capillary or microchannelonto a micro-electrode array would be by methods of flex circuitphotolithography. Accordingly, a photoimageable coverlay material suchas Vacrel 8140®, (a dry film coverlay can be preferred) can be vacuumlaminated onto the gold/plastic laminate. Multiple layers of variouschosen thicknesses can be added to control the depth of the capillarychamber (see infra). The sheet can be exposed to ultraviolet lightthrough a mask pattern to define capillary chambers. The exposedlaminated sheet is developed by conventional methods, e.g., using 1%K₂CO₃, to remove crosslinked photopolymer coverlay material and leavebehind components of a capillary. The sheet is generally thereaftercured, e.g., at 160 C. for 1 hour.

In fabricating the capillary, the depth of the chamber can be controlledby choosing the coverlay material or materials used, according tothickness. Vacrel 8140® film has a thickness of 2, 3, or 4 mil (100 μm).Pyralux PC® 1000, 1500, have 2000 have maximum thicknesses of 2.5 mils(63.5 μm), so double layer lamination gives a chamber depth of 127 μm.Pyralux 1010 has a thickness of 1 mil or 25.4 μm. Capillaries withdepths of greater than or equal to 100 μm have been found to allow fastfill of blood with hematocrits from 20 to 70% to reliably flow into thechamber. Capillary depths of less than 100 microns to 25 microns can beused for other biological fluids such as serum, plasma, interstitialfluid, and the like.

A chemical coating may also be disposed onto the array. First, however,it may be beneficial to clean the sensors. By one cleaning method, asheet of sensors as described can be plasma cleaned in a Branson/IPCPlasma Cleaner according to steps such as the following: (1) O₂ for 1minute at 800 watts; (2) O₂/Argon(Ar) (70/30) for 3 minutes at 220watts; (3) Ar for 2 minutes at 150 watts.

A chemical coating, as described, may be dispensed onto the array, e.g.,into each capillary chamber and over the interdigitated arrays, by knownmethods. The method of dispensing is preferably capable of reproduciblyand consistently delivering very small volumes of a chemicalcomposition, onto the array—e.g., volumes in the range of hundreds ofnanoliters, e.g., 625 nanoliters. As an example, such a coating may bedispensed using known syringe and metering techniques and apparatuses,including dispensing systems sold under the trade name Microdot (fromAstro Dispense Systems, a DCI Company of Franklin, Mass. 02038-9908) andsystems sold by BioDot Inc., Irvine, Calif. The coatings may then bedried of solvent. The inlet ports are opened, and a top foil coated witha hydrophilic adhesive is applied over the capillary chamber using heatand pressure to form the completed three-dimensional sensor structure.

The top foil can be any continuous film capable of defining one side ofthe capillary, and preferably being capable of appropriate processing,e.g., as described herein. Exemplary materials for the foil can includeplastic films such as polyethylene naphthalate (PEN), film type Kadalex1000, 7 mil thick.

Any of a variety of hydrophilic adhesives can be used to bond the topfoil to the sensor. Two part thermoset adhesives such as polyurethanemixtures and isocyanate mixtures can be used, e.g., 38-8668(polyurethane) and 38-8569 (isocyanate) (95:5 wt./wt.) from NationalStarch and Chemical Co. of Bridgewater N.J., or, a two part epoxysystems such as that sold under the trademark Scotch Weld™ 2216 B/A (3MAdhesive Div., St. Paul Minn.), as well as contact adhesives such asthat sold under the trademark Fastbond™ 30-NF Contact Adhesive, providedthat they exhibit acceptable sealing properties to the crosslinkedcoverlay surface. A preferred adhesive was found to be a mixture ofFastbond™ 30-NF Contact Adhesive and the surfactant Triton™ X-100 (UnionCarbide, Danbury Conn.), 93%:7% wt./wt.

EXAMPLES

The following describes a process useful for preparing a sensoraccording to the invention, comprising an interdigitated array disposedon a flexible substrate. According to the method, a gold film can bedeposited onto 7 mil thick Kaladex® film using a planar DC magnetronsputtering process and equipment, from Techni-Met Inc., Windsor, Conn.The thickness of the gold film can range from 30 to 200 nm, with apreferred thickness being 100 nm. Seed layers of chromium or titaniumcan be sputtered between the plastic film and the gold to promote betteradhesion of the gold to the plastic substrate, however, gold layerssputtered directly onto the plastic film can exhibit sufficientadhesion.

The interdigitated array and connectors can be fabricated using batchphotolithography processes common to the flex circuit industry.Electrodes with combinations of finger width and spacing between fingersin the range of 21 to 50 um were easily fabricated using theseprocesses. A preferred configuration of the array was 21 total fingers(10 working electrode fingers and 11 counter electrode fingers), withfinger dimensions of 25 microns (width) by 1 millimeter (length), with21 micron spacing between the fingers.

After the gold was applied to the flexible substrate, a dry filmphotopolymer resist was laminated to the gold/plastic film. A dry filmresist such as that sold under the trademark Riston® CM206 (duPont) waspreferred. The Riston® CM206 photoresist was first wet laminated ontothe gold surface of 12″×12″ gold/plastic panels using a HRL-24 hot rolllaminator (from duPont). The sealing temperature and lamination speedwere 105° C. and 1 meter per minute. The laminated panel was placed in aTamarack model 152R exposure system, from Tamarack Scientific Co., Inc.,Anaheim, Calif. The release liner was removed from the top surface ofthe photoresist. A glass/emulsion photomask of the IDA configuration wasproduced by Advance Reproductions Corporation, North Andover, Mass. Theemulsion side of the mask was treated with an antistick coating(Tribofilm Research Inc., Raleigh, N.C.), and was placed directly ontothe photoresist surface of the panel. The laminated panel was exposed toultraviolet light of 365 nm through the photomask using an exposureenergy of 60 mJ/cm². Exposed photoresist was stripped from the panel ina rotary vertical lab processor (VLP-20), Circuit Chemistry Equipment,Golden Valley, Minn., using 1% potassium carbonate, at room temperature,for 30 seconds using a nozzle pressure of 34 psi. Exposed gold on thesheet was then stripped using an etch bath containing a solution of: 4parts 12:1 part KI:40 parts water vol./vol.; and 0.04 gram Fluorad™fluorochemical surfactant FC99, (3M, St. Paul, Minn.) per 100 gramsolution, added to the bath to ensure wetting of the photoresist. Airwas bubbled through the bath during the etch process to obtain uniformagitation of the bath mixture. The panel was rinsed with deionized waterand residual Riston® CM206 was removed in a 3% KOH bath.

Sensor chambers were fabricated using dry film photoimageable coverlaymaterials such as that sold under the trademark Vacrel® 8140 (duPont) orPyralux® PC series (duPont). The chamber dimensions can be accuratelydefined by flex circuit photolithography. Depth of the chamber wascontrolled by the thickness of the coverlay materials used, and whethersingle or multiple layers of the coverlay dry film were used. Apreferred chamber depth was 125 microns (5 mil). This chamber depth wasachieved by sequential lamination of different coverlay materials asfollows: three mil thick Vacrel® 8130 was first laminated to theelectrode side of the substrate using a HRL-24 (duPont) heated rolllaminator at room temperature, using a roller speed of 1 meter perminute. The electrode panel was then vacuum laminated in a DVL-24 vacuumlaminator (duPont) using settings of 120° F., 30 second vacuum dwell,and a 4 second pressure to remove entrapped air between the coverlayfilm and the electrode substrate. Two mil thick Vacrel 8120 waslaminated next to the Vacrel® 8130 surface using the HRL-24 at roomtemperature, with a roller speed of 1 meter/min. The panel was thenvacuum laminated again in the DVL-24 vacuum laminator using a 30 secondvacuum dwell, 4 second pressure, to remove entrapped air between the twocoverlay films.

The laminated electrode sheet was placed in the Tamarack 152R system andwas exposed to ultraviolet light at 365 nm through the photomask for 22seconds using an exposure intensity of 17 mW/cm². The artwork for thecapillary chamber was a 1 millimeter by 4 millimeter rectangle centeredover the electrode finger array and starting 1 millimeter below thefingers. The exposed coverlay was stripped from the panel to reveal thesensor chamber rectangle using the VLP-20 Circuit Chemistry Equipment)in 1% K₂C0₃, at 140° F., for 75 seconds using a nozzle pressure of 34psi. The developed laminate structure was rinsed in deionized water, andthen cured at 160° C. for 1 hour to thermally crosslink the coverlaymaterial. This completed the construction of the sensor base.

The panel of the base sensors was plasma cleaned to remove residualphotoresist and coverlay material from the exposed gold surface of theinterdigitated array structure. The panel was placed in a barrel etcher,a Barnstead/IPC model P2100 from Metroline/IPC of Corona, Calif. Thepanel was first exposed to an oxygen plasma for 1 minute at 800 wattsand 1.1 torr pressure to oxidize the panel surface. It was then etchedin an oxygen/argon plasma mixture (70/30 vol./vol.) for 3 minutes, at225 watts and 1.5 torr pressure, and was finally stripped in an argonplasma for 2 minutes, at 150 watts and 2 torr pressure.

The chemical coating was formulated for measurement of d-glucose in ahuman blood sample. The chemical coating was reactive with the sample ina manner effective to generate an electrical output signal indicative ofthe level of glucose in the sample. The coating included a mediator,enzymes, and a cofactor. The coating further comprised film formingagents and detergents conferring durability and providinghydrophilicity. The ingredients are listed in Table 1; unless statedotherwise, all concentrations refer to the concentration of a givensubstance in a wet-coating, prior to the deposition and drying of thecoating onto the array.

The chemical coating was formulated from several sub-mixtures ofcomponents. A first mixture contained glycerophosphate buffer, from ICNBiomedicals Inc. Aurora, Ohio; Medium Viscosity Alginic acid, from SigmaChemical Co., St. Louis, Mo.; Natrosol 250M, from Hercules Inc.,Wilmington, Del.; and Triton® X-100, from Union Carbide, Danbury Conn.These components were added to a volume of distilled water sufficient tomake a 250 gram solution of the buffer/polymer/surfactant (see Table 1).The solution was mixed overnight to allow complete hydration of theNatrosol and Alginic acid. The pH of the completed solution was adjustedto 6.9 to 7.0 with concentrated hydrochloric acid. This solution isknown hereinafter as “Solution A.”

A second solution prepared was a concentrated enzyme/cofactor matrix.8.2 milligrams pyrrolo-quinoline-quinone (PQQ), Fluka, Milwaukee, Wis.,was added to 25.85 grams of Solution A. The resulting mixture wassonicated until the PQQ was completely in solution. 1.1394 grams of theenzyme, Glucose-De-oxidoreductase (GlucDor), from Roche MolecularBiochemicals, Indianapolis, Ind., was added to the solution. The finalmixture was rocked for 2 hours to allow formation of the GlucDor/PQQholoenzyme. The completed solution will be referred to as “Solution B.”

Potassium ferricyanide was added to the composition as follows: 4.4173grams of potassium ferricyanide, from J. T. Baker, Phillipsburg, N.J.,was added to 70.58 grams of Solution A. The resulting solution was mixeduntil the ferricyanide was completely in solution. The completedsolution will be referred to as “Solution C.”

The final coating composition was completed by combining 63 grams ofSolution C to 25 grams Solution B. This composition was rocked in thedark for 1 hour to thoroughly mix.

TABLE 1 Formulation per 100 grams of coating Concentration/ Wet Drymass/sensor Component activity mass (g) (mg) Distilled Water 88.487Disodium 150 mM 4.359 0.0287 Glycerophosphate pH 6.98 Trehalose 1% wt/wt1.000 0.0066 Natrosol 0.3% wt/wt 0.300 0.002 Alginic acid 0.4% wt/wt0.400 0.0026 Medium viscosity Triton X-100 0.025% wt/wt 0.025 0.00016Pyrrolo-quinoline- 0.261 mM 0.0082 5.3382 × .10 − 5 Quinone (PQQ)GlucDor Enzyme 2034 u/mg 1.1394 0.0075 15.23 (units) Potassium 137 mM4.2814 0.0281 Ferricyanide

A preferred method for applying the chemistry matrix to the sensorchamber (IDA) is a discrete dispense of 500 nanoliters of the coatingsolution into the 1 millimeter×4 millimeter chamber using amicrodispensing system such as that sold under the trademark of BioJetQuanti3000™, BioDot Inc., Irvine, Calif. The coating covered both theworking and counter electrodes of the IDA. The coating was dried for 1.5minutes at 45° C. in a horizontal air flow oven, VWR ScientificProducts, Chicago Ill.

The hydrophilic top foil was prepared by coating an adhesive mixture(e.g., a mixture of Fastbond™ 30-NF Contact Adhesive and the surfactantTriton™ X-100 (Union Carbide, Danbury Conn.), 93%:7% wt/wt.) to a wetthickness of 25 um onto 5 mil polyester film such as that sold under thetrademark Melinex® “S” (duPont Polyester Films, Wilmington Del.) using awire bar coater from Thomas Scientific, Swedesboro, N.J. The coated topfoil was dried for 2 minutes at 50° C. in a horizontal air flow oven(VWR Scientific Products). The capillary chamber was opened by cutting 1millimeter in from the front edge of the capillary chamber with a pairof scissors. The dried coated top foil was applied to the sensor,allowing approximately a 0.5 mm space between the back edge of thechamber and the edge of the top foil as an air vent. The top foil wassealed to the sensor surface using a 5 ton press with a heated topplaten, at 81° C., 60 psi for 5 seconds. The panel of completed sensorswas cut into individual sensors and stored desiccated at 8% RH untiltested.

The sensors were evaluated using chronoamperometry electrochemicaltechniques on test stands such as that sold under the trademark ofBAS™100W Electrochemical Workstation, Bioanalytical Systems, Inc. WestLafayette, Ind. The preferred electrochemical test stand used in theevaluation of the electrodes was a dedicated test stand for DCchronoamperometric current measurement for assay potentials from ±1volt.

The sensors may be used to determine the concentration of an analyte,such as glucose, in a fluid sample by performing the following steps:

Set up the test stand parameters:

In accordance with a “drop detect” system, an initial potentialdifference is established between the working and counter electrodes−300 mV (millivolts)—to start timing of the analysis sequence. Currentresponse to this potential is triggered by contact of the array with afluid sample.

The initial current response upon application of the test solution tothe sensor chamber is generally greater than 0.4 microamps.

The time (delay period) between the threshold trigger and re-applicationof the 300 mV potential difference (assay potential) is generally 3seconds.

The assay period, after re-application of the 300 mV potentialdifference between the working and counter electrodes of the sensor isgenerally 9 seconds.

In more detail:

Insert the sensor into the test stand connection. Apply approximately0.3 uL of a fluid sample to the opening of the capillary chamber. Fluidwill flow into the chamber by capillary action covering the chemicalcoating applied to the working and counter electrodes. The thresholdcurrent will be triggered when the sample fluid covers the nearestworking and counter electrode fingers. Once triggered, the potentialdifference will go to open circuit for a 3 seconds, during the delayperiod.

During the delay period, reaction will occur between the reactants(analyte, enzyme/cofactor, and the oxidized form of the mediator),resulting in reduction of the mediator.

The 300 mV assay potential difference is re-applied between theelectrodes after the 3 second delay. This causes electro-oxidation ofthe reduced mediator at the surface of the working electrode.

The current/time reaction profiles of the assay show a characteristicpseudo-steady-state current/time plateau starting 0.5 to 1.5 secondsafter re-application of the 300 mV assay potential to the sensor.Currents at fixed assay period points chosen in this plateau region wereproportional to the concentration of analyte in the sample fluid. Assayendpoints were chosen in such a manner give a linear dose response forglucose concentrations from 0 to 600 mg/dL. See FIG. 7.

1. A method of determining the concentration of an analyte in a liquidsample comprising: providing a test sensor including a sample receivingcavity having a volume of less than about 1.5 μl, the sensor includingat least one working electrode and at least one counter electrodedisposed within the sample receiving cavity, the sensor furtherincluding a reagent layer comprising an enzyme and a mediator, thereagent layer being disposed on at least the working electrode, at leastone of the enzyme and mediator being selected to react with the analyteto generate an electrochemical signal representative of theconcentration of the analyte in the liquid sample; admitting the liquidsample into the sample receiving cavity; detecting the time at which theliquid sample enters the sample cavity; following said detecting,controlling the voltage or current across the working and counterelectrodes; and within ten seconds of said detecting, obtaining areadout of the concentration of the analyte in the liquid sample.
 2. Themethod of claim 1 in which the sample receiving cavity has a volume ofless than or equal to about 1.0 μl.
 3. The method of claim 1 in whichthe sample receiving cavity has a volume of between about 0.1 μl andabout 1.5 μl.
 4. The method of claim 3 in which the sample receivingcavity has a volume of between about 0.6 μl and about 1.0 μl.
 5. Themethod of claim 1 which includes obtaining a readout of theconcentration of the analyte within about 6 seconds after saiddetecting.
 6. The method of claim 5 which includes obtaining a readoutof the concentration of the analyte between about 2 seconds and about 6seconds after said detecting.
 7. The method of claim 6 which includesobtaining a readout of the concentration of the analyte between about 2seconds and about 5 seconds after said detecting.
 8. The method of claim7 which includes obtaining a readout of the concentration of the analytewithin about 5 seconds after said detecting.
 9. The method of claim 1 inwhich said controlling comprises applying a DC voltage of 100-500 mVacross the working and counter electrodes, said obtaining comprisingmeasuring the current received from the working electrode anddetermining the concentration of the analyte from the measured current.10. The method of claim 9 in which said applying a voltage comprisesapplying a voltage of about 200 mV to about 400 mV.
 11. The method ofclaim 1 in which said obtaining comprises measuring the current receivedfrom the working electrode and determining the concentration of theanalyte from the current within ten seconds of said detecting.
 12. Themethod of claim 1 in which the sample receiving cavity has a height ofabout 25 to about 200 μm.
 13. The method of claim 1 in which the reagentlayer is present as a coating on both the working and counterelectrodes.
 14. The method of claim 1 in which the analyte is glucose.